Many types of internal combustion engines use a carburetor for supplying the engine with properly atomized and vaporized fuel mixed with air. The air-fuel mixture is fed from the carburetor through an intake manifold to the combustion chambers of the engine. A typical carburetor includes a fuel bowl, which holds a measured amount of fuel from a gas tank. The fuel bowl includes a bowl vent, which allows air into the fuel bowl, and a fuel discharge tube for discharging fuel from the fuel bowl into a venturi section of the carburetor. In the venturi section air, which is drawn into the carburetor typically through an air filter or cleaner, is mixed with the fuel discharged from the fuel bowl. A throttle valve, located in the carburetor downstream from the venturi section thereof, controls the amount of air-fuel mixture entering the engine's combustion chambers.
When an internal combustion engine is not in operation, fuel vapors due to evaporation of the fuel in the fuel bowl and other parts of the carburetor or in the engine, can escape to the atmosphere through the bowl vent and/or through the carburetor air intake path. These hydrocarbon vapors, along with emissions of combustion products through the exhaust systems of internal combustion engines, can contribute to air pollution. As a result, many states, most notably California, have adopted evaporative emissions standards for internal combustion engine-powered vehicles, such as automobiles and motorcycles.
Various systems have been developed for limiting the evaporative emissions of hydrocarbons from the carburetor of an internal combustion engine. Such systems typically employ an air intake gating valve positioned upstream of the carburetor fuel discharge tube, either between the venturi section of the carburetor and the air filter, or between the air filter and the outside atmosphere. When the engine is not in operation, the gating valve is closed to prevent evaporative emissions from the carburetor. When the engine is in operation, the gating valve is opened to allow air to be drawn into the carburetor from the atmosphere. Such gating valves are typically controlled using a vacuum-operated valve controller that is connected in fluid communication with the carburetor, typically downstream of the venturi section of the carburetor, or some other part of the intake system such as the intake manifold, such that when the engine is running a negative pressure is induced as the engine draws air through the carburetor, which opens the valve. Typically, the gating valve is mechanically coupled to a diaphragm in the vacuum-operated valve controller that responds to the negative pressure (below ambient pressure) in the carburetor to open the gating valve. Various mechanisms may be employed to ensure that the gating valve is not closed unintentionally due to transient drops in the volume of air drawn through the carburetor when the engine is in operation. Such air intake volume drops may result in temporary loss of the negative pressure level which is used to hold the gating valve open.